1. Field of the Invention
This invention relates generally to a radio wave receiver and, more particularly, to radio receiver including a circuit for detecting the electric field intensity of a received radio wave.
2. Description of the Background
Conventionally, the process of detecting signal strength, that is, the intensity of a received electric field in a tuner is carried out by comparing a signal corresponding to the electric field intensity of the received signal or audio wave to a predetermined threshold value to derive an indicator signal. Because it is generally sufficient for the tuner to indicate only whether the received signal is visible and audible or not, it is not necessary to detect the absolute level of the intensity of the electric field of the received signal.
In one example of such a receiver signal strength indicator, intermediate frequency (IF) carrier components are rectified to derive a direct current (DC) voltage that is compared with a threshold value to obtain a binary signal as a comparison result. It can then be determined from the level of the binary signal that the electric field intensity is high, if the direct current voltage is larger than the threshold value, and low, if the direct current voltage is smaller than the threshold value. This approach can be used, for example, in an automobile radio receiver and tape player to change-over to the radio receiver when the electric field intensity of the received broadcast signal is high, that is, when the binary signal is at a high level and to change over to the tape player for listening to a tape when the electric field intensity of the received broadcast signal is low, that is, when the binary signal is at a low level.
In the above-mentioned system for detecting the electric field intensity of a received signal by using a binary signal of high and low levels, if it is assumed that an auto radio receiver is receiving a broadcast signal from a certain broadcasting station while the car is moving in a direction away from this broadcasting station, the electric field intensity of the received broadcast signal will change in some fashion, for example, as shown in FIG. 1A. In such case, even though the electric field intensity is high, if the car passes in front of a high building, the electric field intensity will instantaneously become lower, as shown in FIG. 1A at a time t.sub.1. Thus, the binary signal moves from the high level to the low level, as shown in FIG. 1B, to consequently halt the operation of the receiver and change-over to the tape player in response to this transition of the binary signal caused by the large building. Subsequently, at some time t.sub.2, when the car has passed the large building, the binary signal will again go high thereby to change over once again and resume operation of the receiver.
In another situation, when the electric field intensity becomes lower and the signal moves from a high electric field intensity to the vicinity of the threshold level, there is a tendency that noise will increase relative to the signal with the lowered electric field intensity. As a result, the electric field intensity varies readily between higher and lower values relative to the threshold level, as shown in the time period between times t.sub.3 and t.sub.11 in FIG. 1A, so that the binary signal correspondingly moves frequently and hence it it difficult to remain set at one of the high and low levels. In other words, the system appears unstable.
In a different situation, an FM multiplex system called Radio Data System (RDS) is planned to be introduced in Europe. In the RDS system a subcarrier of 57 kHz is provided above the FM base band for multiplexing approximately 1.2 kbps of digital information as a two-phase phase-shift keying (PSK) signal. One use for this RDS signal is to transmit data identifying another broadcasting station which is transmitting the same program at the same time, of course, the tuning value is offset from the broadcasting station presently tuned. More than one such station could be identified.
It is therefore possible to automatically tune to one of the other broadcasting stations by the use of the above-mentioned RDS data if a receiving condition becomes worse in a radio receiver mounted in a travelling body such as a car, thereby to maintain the same program information being presented to the listener.
With this function, the timing of changing over from one broadcasting station to another may be determined by detecting the electric field intensity of the broadcasting signal presently being received.
Assuming that the above-described discriminating method using the binary signal is employed for the detection of the electric field intensity, it is probable that the electric field intensity will be instantaneously lowered, or the electric field intensity will be varied in the vicinity of the threshold level, to frequently change-over the receiver to the frequency of the other broadcasting station, so that a beat may be generated every time the change-over is effected or the volume may suddenly be increased if the electric field of the signal being transmitted from the newly acquired broadcasting station is too strong. If such noises occur frequently, the listening conditions will be intolerable for travellers in the car.
Alternatively, if the absolute value of the electric field intensity of a received signal is digitized and employed for electric field discrimination, the above-mentioned problem could be eliminated. Nevertheless, because such digital data would require multiple bits for example, 8 bits, the circuit for discriminating such multiple bit data would be complicated and expensive.